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This chapter reviews fundamental properties and recent advances of diffuse and pulsating aurora. Diffuse and pulsating aurora often occurs on closed field lines and involves energetic electron precipitation by wave-particle interaction. After summarizing the definition, large-scale morphology, types of pulsation, and driving processes, we review observation techniques, occurrence, duration, altitude, evolution, small-scale structures, fast modulation, relation to high-energy precipitation, the role of ECH waves, reflected and secondary electrons, ionosphere dynamics, and simulation of wave-particle interaction. Finally we discuss open questions of diffuse and pulsating aurora.

The early-morning auroral oval is dominated by pulsating auroras. These auroras have often been discussed as if they are one phenomenon, but they are not. Pulsating auroras are separable based on the extent of their pulsation and structuring into at least three subcategories. This study surveyed 10 years of all-sky camera data to determine the occurrence probability for each type of pulsating aurora in magnetic local time and magnetic latitude. Amorphous pulsating aurora (APAs) are a pervasive, nearly daily feature in the early-morning auroral oval which have an 86 % chance of occurrence at their peak. Patchy pulsating auroras (PPAs) and patchy auroras (PAs) are less common, peaking at 21 % and 29 %, respectively. Before local midnight, pulsating auroras are almost exclusively APAs. Occurrence distributions of APAs, PPAs, and PAs are mapped into the equatorial plane to approximately locate their source regions. The PA and PPA distributions primarily map to locations approximately between 4 and 9 RE, while some APAs map to farther distances, suggesting that the mechanism which structures PPAs and PAs is constrained to the inner magnetosphere. This is in agreement with Grono and Donovan (2019), which located these auroras relative to the proton aurora.

There is mounting evidence which suggests that pulsating auroral patches often move with convection. This study is an initial step at identifying the differences between patches that move with convection and those that do not. While many properties of pulsating patches vary, here we outline criteria for separating pulsating auroral patches into three categories based on two properties: their structural stability and the spatial extent of their pulsations. Patchy aurora is characterized by stable structures whose pulsations are limited to small regions. Patchy pulsating aurora consists of stable patches whose pulsations are far less subtle and occur throughout much of their area. Amorphous pulsating auroral structures are unstable – very rapidly evolving – and can pulsate over their entire area. The speed with which amorphous pulsating aurora evolves makes their motion difficult to ascertain and seems unrelated to the E×B drifting of cold, equatorial plasma.

Pulsating aurora is frequently observed in the evening and morning sector auroral oval. While the precipitating electrons span a wide range of energies, there is increasing evidence that the shape of pulsating auroral patches is controlled by structures in near-equatorial cold plasma; these patches appear to move with convection, for example. Given the tremendous and rapidly increasing amount of auroral image data from which the velocity of these patches can be inferred, it is timely to develop and implement techniques for the automatic identification of pulsating auroral patch events in these data and for the automatic determination of the velocity of individual patches from that data. As a first step towards this, we have implemented an automatic technique for determining patch velocities from sequences of images from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) all-sky imager (ASI) and applied it to many pulsating aurora events. Here we demonstrate the use of this technique and present the initial results, including a comparison between ewograms (east–west keograms) and time series of patch position as determined by the algorithm. We discuss the implications of this technique for remote sensing convection in the inner magnetosphere.